The present invention concerns a catalyst and a process for the isomerization of an aromatic C8 cut. The catalyst of the invention is an aluminosilicate comprising a mordenite zeolite in which the selectivity and/or catalytic properties has been modified by selectively deactivating the external crystal surface by forming a layer of cerium oxide on the surface, and comprising at least one metal from group VIII of the periodic classification of the elements (Handbook of Chemistry and Physics, 65th edition, 1984-85), and a matrix. In general, the external surface of zeolite crystals is modified by dry impregnation of mordenite by an alcoholic or aqueous solution of a cerium salt.
Commercial catalysts currently used for the isomerisation of aromatic C8 cuts are essentially based on ZSM5 zeolite (U.S. Pat. No. 4,482,773, for example).
ZSM5's importance lies in its excellent form selectivity, leading to high selectivity for para-xylene; the selectivity regarding undesirable secondary dismutation reactions remains at a very low level.
Other zeolites with larger pore openings of 12MR (an opening the size of 12 oxygen atoms) have also been used, such as mordenite. Mordenite based catalysts are particularly described in U.S. Pat. No. 4,723,051. However, these zeolites do not have particular geometrical selectivity properties. This means that, whatever its Si/Al ratio, para-xylene selectivities are lower than those obtained for ZSM5 zeolite and in particular, leads to higher production of trimethylbenzene. The production of trimethylbenzenes by dismutation is actually encouraged in mordenite where the microporous system is more open than that of ZSM5: the openings are the size of 12 oxygen atoms instead of 10 for ZSM5.
We have researched improved catalysts based on mordenite to reduce side reactions, in particular the production of trimethylbenzene.